There are a wide variety of fields which utilise laser light. A particular application is in the treatment of human cancer tumours. It has been established that illumination of tumours previously labelled with suitable photoactive substances (for example, HPD - haematoporphyrin derivative) is an effective method of treating such tumours. The wavelength (colour) of the illumination light must be such that interaction with the photoactive substance occurs while at the same time adequate penetration of the tissue is permitted. In the case of the drug HPD, for example, photoactivity occurs for wavelengths in a narrow band (.+-.5 nm) centred at 630 nm while good transmission of light through haemoglobin and thus through the tissue is obtained for wavelengths greater than 600 nm.
Both continuous and pulsed light have been used for the above purpose. Continuous light sources employed to date have been conventional arc discharge lamps of high intensity used with appropriate filters or a continuous dye laser pumped by an argon ion laser. Pulsed light of the desired wavelength and average power can be obtained using a gold vapour laser, which operates directly within the required band at 628 nm or a pulsed dye laser pumped by a pulsed copper laser and whose wavelength is tuned in the required band. Given that fibre-optics are required to deliver the light to tumour sites within the body, laser sources are preferred over conventional sources (for the puposes of efficient coupling of light into the fibres). Moreover there is evidence that pulsed lasers are superior to continuous lasers in therapeutic effect.
Continuous dye lasers pumped by argon ion lasers have been found to have considerable disadvantages in practice for the above application. These systems are complex requiring delicate alignment which can alter over time and are therefore unreliable for routine use; they are also very expensive to install and maintain and electrically extremely inefficient.
The gold vapour laser, though generating emission directly within the required band at the appropriate energy densities, also has significant disadvantages in practice. The very high operating temperatures, greater than 1750.degree. C., required of the plasma tube place great stress on the construction materials; the low energy conversion efficiency (compared with the copper vapour laser) results in stress on the high-voltage electrical excitation circuitry and components and places added requirements on cooling facilities; and the laser material (gold) is expensive in the quantities required. All these factors combine to make the gold laser also unreliable in operation and expensive to install and maintain.
Pulsed dye lasers based on the pulsed copper laser as a pump source have considerable advantages over both the argon-pumped dye laser and the gold vapour laser. In the first place the copper laser pump source itself is a practical and reliable device (with plasma tube operating temperatures only about 1500.degree. C.), relatively cheap to install and maintain and having good electrical energy conversion efficiency. Second, the efficiency of conversion of the copper laser pump power (at wavelengths in the green and yellow) to high-pulse-rate dye laser output (at wavelengths in the orange, red and infrared) is high, up to 50%, in direct conversion amplifiers. However to achieve such high conversion efficiencies a dye amplifier must be injected with an optical signal at the appropriate wavelength within the amplification band of the dye and with sufficient initial power. Such an injection signal is normally provided by a dye oscillator also optically pumped with a portion of the copper laser pump power. Copper-laser-pumped dye oscillators are themselves relatively inefficient (&lt;15%) in converting the pump laser power to dye oscillator output. The optical arrangements of dye oscillators are usually complex including a loss causing frequency-selective element (often a diffraction grating) as part of the optical cavity, and, in many cases, additional optical components (lenses or prisms) to expand the optical beam at the grating. Although such dye oscillators have the advantage that the operating wavelength is tunable over the amplification band of the dye used in the oscillator, the tolerance to misalignment can be low and, in applications where a fixed operating wavelength is required, wavelength and power stability are difficult to achieve. Moreover the optical quality of such dye oscillator output beams is low, often requiring very inefficient beam clean-up techniques to be applied before injection into the subsequent dye amplifier(s).
It follows from above that there is a need for a wavelength locked pulsed laser light source where the wavelength of the laser light is near 630 nm. It is the object of the invention to provide a wavelength locked pulsed laser with high average power at good overall efficiencies (&gt;20%) and with low beam divergence (&lt;1 mrad).